PROPAGATIONIonosphere and solar activity

Propagation

O - the study of the movement of radio

waves through various mediums (air,

ground, water, etc.)

Some FundamentalsO All radio waves are the result of the acceleration of

charged particles: f=ma

O Refraction is the bending of radio waves as a result of

moving from one medium to another

O Radio waves generally attenuate with distance (inverse

square law)

O Polarization of an electromagnetic wave is defined as the

polarization of the electric field

O Electric field is the same polarization as the antenna

Ionosphere

O Starts at about 60 miles in altitude and

ends at about 500 miles

O It consists of layers but the lines of

demarcation between and within layers is

a but fuzzy

O In a neutral atom there are equal charges

(positive and negative), so there is n net

charge for an intercepting radio signal to

interact with

Ionosphere (cont.)

O To prepare a particle to respond to a , the

radio signal, the electrons must be

separated from the protons (or the

nucleus) – a process called ionization

O The primary ionizing agent is radiation

from the sun, primarily ultraviolet (UV)

O Not all atoms are ionized, so we have a

mixture of ions and neutrals in the

ionosphere

Ionosphere (cont.)

O The greatest amount of ionization should

occur on the side facing the sun and at

high noon

O There is a high degree of correlation

between the height of the sun and the

maximum usable frequency (MUF)

O The unpredictability of ionospheric

propagation is due to the mixture of ions

and neutrals and their interaction

Ionosphere (cont.)

O Air pressure is dependent on altitude so particle density decreases as we move upward

O Since ions come from air, ion density decreases with altitude

O At high altitudes were air pressure is very low its easy to create ions (don’t need a lot of UV)

O Because the ionosphere is a refractor, not a reflector, the speed of a radio wave decreases as it penetrates the ionosphere so it appears that the reflection point is higher than it actually is

Layers of the Ionosphere

O Below the ionosphere there are three layers (part of our atmosphere)

O Troposhpere (A layer)

O Stratosphere (B layer)

O Mesosphere (C layer)

O These layers are not ionized (radio waves go right through)

O The ionosphere consists of three layers: D, E and F

IONOSPHERE REGIONS

THE IONOSPHERIC LAYERS

The D layer (absportion layer): is the innermost layer, 50 km to 90 km above the surface of the Earth. when the sun is active with 50 or more sunspots, During the night cosmic rays produce a residual amount of ionization as a result high-frequency (HF) radio waves aren't reflected by the D layer. The D layer is mainly responsible for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime.

The E layer: is the middle layer, 90 km to 120 km above the surface of the Earth. This layer can only reflect radio waves having frequencies less than about 10 MHz. It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. At night the E layer begins to disappear because the primary source of ionization is no longer present. The increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.

The F layer: or region, is 120 km to 400 km above the surface of the Earth. It is the top most layer of the ionosphere. Here extreme ultraviolet (UV) (10-100 nm) solar radiation ionizes atomic oxygen (O). The F region is the most important part of the ionosphere in terms of HF communications. The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2. The F1 layer consists of oxygen and nitrogen which don’t ionize easily. F2 consists of helium, hydorgen and noble gases which are ionized more easily, so the F2 is available for more hours than the F1.The F layers are responsible for most skywave propagation of radio waves, and are thickest and most reflective of radio on the side of the Earth facing the sun.

Ionospheric Layers

• During the daytime, in periods of high

solar activity, the F region splits into two

regions

• F2 remains thicker and plays the

predominant role in HF communications

• As the sun goes down, the F region

retreats from the bottom up (F1 goes away

first and the band “goes long”

Ionogram

• Available at:

www.digisonde.com/stationlist.php

• A graph generated by an ionosonde,

which send short RF pulses straight up

and receives the returning signal after

reflection from the ionosphere

• The X-axis is the frequency and the Y-axis

is the reflection height (km)

Ionogram (cont.)

• When the curve becomes vertical, this is

the critical frequency – the maximum

frequency that will be reflected to earth for

a vertical incident signal (not the same as

MUF but they are related)

• foF2 is the critical frequency for the o-

mode (F layer). There is also an E for the

E-layer

• At the bottom, D is the distance per hop

(km) – MUF increases proportionately to D

Ionogram (cont.)

• Distance per hop is a function of the

vertical launch angle and the reflection

height

• MUF is dependent on the angle of the

wave penetrating the ionosphere

• FxI is the critical frequency for the X-mode

Solar Activity

• The Sun’s atmosphere contains three

parts:

– Photosphere

– Chromosphere

– Corona

• The outer layer of the Sun contains a

convection zone where hot gases are

moved upward and downward on their

way to the surface

Sunspots

• The surface of the Sun consists of

granules – rising columns of hot gas

surrounded by dark lanes which are cooler

gas descending into the interior (last about

15 minutes, but 40% seem to explode,

expanding horizontally)

• Sunspots are cool relatively dark on the

photosphere caused by magnetic fields

which become bent by differential rotation

Sunspots

• They are between 2500 km to 50,000 km

in diameter and last from less than an hour

to several months

• They are generally found in groups

• They have a dark central core – the

umbra, and a brighter border – the

penumbra. The umbra is about 4300 K

and the penumbra is 5000 K (photosphere

is 5800 K

Sunspots (cont.)

O The area around the sunspot is called an active region because flares and other events occur there

O Sunspots are associated with intense magnetic fields on the Sun

O The solar atmosphere is a gaseous mixture called plasma where elecricallycharged ions and electrons move freely and are deflected by the magnetic field resulting in a region where the gas is relatively cool

Sunspots (cont.)

O The Sun rotates at different speeds (called

differential rotation). At the equator it takes 25

days for a complete rotation while at the poles

it takes 36 days

O This differential rotation causes the magnetic

field in the photosphere to become wrapped

around the Sun (east to west)

O As a result, the magnetic field becomes

concentrated at certain latitudes on either side

of the equator

Sunspots (cont.)

O Convection in the photosphere creates

tangles in the concentrated magnetic field,

and “kinks” erupt through the solar surface

O Sunspots appear where the magnetic field

protrudes through the photosphere

O Sunspots go through a somewhat regular

11-year cycle.

O This cycle can be explained by the

Babcock model

Babcock Model

O Starting from sunspot minimum, the

magnetic field of the Sun is weak

O Magnetic lines emerge at latitudes greater

than 55 degrees and connect back to the

opposite polar region

O Since the rotation period is shorter at the

equator, the equatorial field lines become

stretched and twisted which increases the

magnetic field strength

Babcock Model (cont.)

O Eventually twisted ropes of magnetic field

lines erupt as loops through the Sun’s surface

and produce regions where sunspots appear

O Near the end of the cycle, the subsurface

magnetic field is stretched and twisted so

much that active regions of sunspot groups

form near the equator

O Groups from the north and south have

opposite magnetic polarity, so when they meet

at the equator, they cancel each other out

Babcock Model (cont.)

O The Sun’s polarity pattern completely

reverses itself every 11 years

O The hemisphere that has preceding north

magnetic poles during one 11-year cycle

will have preceding south magnetic poles

during the next cycle

O North and south magnetic poles of the

Sun reverse every 11 years (thus a 22-

year cycle)

Sunspots and Propagation

O Ultraviolet radiation from the sun is the chief (though not the only) source of ionization in the upper atmosphere. During periods of low ultraviolet emission the ionization level of the ionosphere is low and radio signals with short wavelengths will pass through and be lost to space. During periods of high ultraviolet emission higher levels of ionization reflect higher frequencies and shorter wavelengths will propagate much longer distances

O The higher the solar activity, the stronger the ionization of the F-layer. A strong ionization of the F-layer increases its reflecting power. Stronger the ionization, the higher the maximum usable frequency (MUF), exceeding regularly 40 or 50 MHz in such occasions.

Numbers

O SF = 73.4 + 0.73R + 0.0009R2

O K index scale is 0-9 (prefer 0 or 1)

O A index scale is 0-400 (prefer 0-4)

O Charged particles are generally disruptive

to propagation – best with high solar flux

and low magnetic activity

Software

• www.arrl.org/propagation

• www.dxzone.com/catalog/Software